1. Field
Exemplary embodiments of the present invention relate to a light emitting apparatus, a planar surface light source apparatus, and the luminous flux control or a deterministic light deviating member more commonly known as an optical lens.
2. Discussion of the Background
In the past decade, flat-screen televisions have gone from luxury-status to market domination that is so complete that cathode-ray tube (CRT) televisions have ceased production and old CRT sets cannot be given away. While plasma flat-screens are like CRTs in having emissive pixels, the pixels of LCDs are passive, merely acting to rotate the polarization of light passing through them, so that they must be put between two orthogonal polarizers in order for the pixels to act as intensity modulators. Originally, flat-screen LCDs provided this intensity by utilizing waveguide-based backlights that were edge-illuminated by fluorescent tubes. As LEDs rapidly increased in brightness and efficacy, they replaced the fluorescent tubes while retaining the edge-illuminated waveguide. However, waveguides may be thick and heavy for large screen sizes, so that direct-view area backlights comprising hollow light boxes may be desirable because their lights are distributed all across the back of the light box. Direct-view area backlights may have to spread the light out uniformly to eliminate hot spots on the screen just over each LED light source, which is what LEDs without dedicated local lenses may not accomplish.
As LEDs have matured, their increasing power output means that fewer and fewer LEDs may be needed to do any particular illumination task. When area backlights try to use fewer LEDs, especially with the 16:9 proportions of typical high-definition televisions, their illumination geometry may become increasingly difficult to achieve uniform illumination in several ways.
First, locations intermediate between the LEDs are subject to a cosine to the third-power drop-off, which even at 60° off-axis is an 8:1 ratio. This may be over and above the lesser lateral intensity typical of LEDs. Second, an illumination lens may be subject to inevitable scattering, due to imperfections inherent to the injection-molded lens material, that becomes more important as LED luminosity increases. This may superimpose hot spots upon the direct-light pattern which the lens produces, requiring compensation of that pattern with a central dark zone.
In some applications, these adverse factors may be dealt with by increasing the relative size of the illumination lens compared to that of the LED. LCD backlights, however, may be an inch or less in thickness, severely limiting lens size. Also, this thin geometry increases the strength of light scattering by the lens.
LED backlights are used for myriad applications including LCDs, reach-in refrigeration lighting, and general illumination (light fixtures). All of these applications have a throw distance (panel thickness) much less than the spacing between the LED sources. Since LEDs emit in a quasi-Lambertian manner, a diverging lens is used to spread the emitted light out across a large lateral area. This results in the need for a lens that can modify the forward emitting angular distribution of the LED source into a mostly side-emitting angular distribution. The conventional art has focused almost exclusively on solutions with rotational symmetry. While some of these solutions predict near ideal uniformity in one dimension, they are fundamentally limited by étendue, specifically its skewness, from achieving near ideal uniformity in two dimensions.
In LED backlights, the circuit boards holding the LEDs may be separated by a span greater than the pitch of the LEDs on the board. This may require the illumination lens to produce an asymmetric or rectangular pattern, which adds further difficulties due to the topological incompatibility of the nearly round pattern of the LED versus how light must be horizontally deflected towards the long end of the rectangle.
The array of illumination lenses may superimpose their patterns for intermediate locations, but may create a potential deficit of illumination around the edge of the backlight. The light box may include a specular minor or 45° beveling around its edges, but for cost reasons this may be impractical. Backlights may employ the 45° beveling around the perimeter, which will dictate how few LEDs can be used and still attain illuminance uniformity.
The conventional art discloses how to construct a surface light source device using a plurality of LEDs as the lighting means for backlighting an LCD monitor for use in personal computers, LCD television sets, tablet displays, as well as for smart phones. U.S. Pat. No. 7,798,679 discloses an example of this architecture. The surface light source device taught therein uses a plurality of LEDs with lenses designed to deviate the light emitted from the LEDs in a deterministic manner; moreover, these lighting elements are situated in a planar array having substantially the same shape as the LCD panel that it is to illuminate. This surface light source device then illuminates the LCD device from the LCD's back surface side.
The '679 patent goes on to disclose that when one deviates the light emitted from the LEDs in the 50°-70° range in order to achieve good uniformity on the LCD screen, one may be confronted by unwanted Fresnel reflections at the base of and interior to the lens outer perimeter. This may lead to secondary bright sources that cause non-uniformity in the LCD backlight display and can be compensated to a certain extent by incorporating a faceted structure to randomly scatter this unwanted light from bottom surface of the lens.
U.S. Pat. No. 8,227,969 considers integrating various types of light scattering bottom surface features and how to construct the faceted bottom surface of flux controlling and light diverting lenses. Convex and concave facets of linear and diagonal geometries are modeled as well as pyramidal scattering facets, including rough etched surfaces on these various facet types. The '969 patent discloses that without an effective diffusing bottom of the lens feature, refractive lenses cannot yield good LCD screen uniformity.
U.S. Pat. No. 8,328,395 discloses the importance of the size and placement of lens leg attachments to a printed circuit board (PCB) along with the facet treatment to defeat strong Fresnel reflections. The '395 patent discloses strategically placing leg attachments to very specific spatial positions and employing those legs as optical conductors to remove some of the unwanted Fresnel reflections, while even including blackening these legs to increase unwanted light absorption, for axial-symmetric lens shapes.